The production of polyfunctional uretdione isocyanates (for example, based on 2,4-tolylene diisocyanate) using dimerization catalysts, is known (Kunststoff-Handbuch, Vol. VII, Polyurethane, published by Vieweg-Hochtlen, Carl-Hanser-Verlag, Munich, 1966, page 16). In one embodiment, dimerization is carried out in an organic solvent such as dichlorobenzene (U.S. Pat. No. 2,683,144). In order to obtain high quality on a commercial scale, it has been found necessary to use higher diluted solutions. It is obvious that the use during dimerization of the large quantities of solvent which this involves and which have to be removed on completion of the reaction, involves very considerable outlay.
In another embodiment, dimerization is carried out in aqueous medium. According to U.S. Pat. No. 3,489,744, dispersants such as alkyl aryl sulphonic acids or the alkali metal salts of higher organic acids, are used. The disadvantage of this process is that a considerable proportion of the diisocyanates react with water to form ureas and polyureas, so that the products consist of a mixture of diisocyanates containing uretdione and urea groups. Additionally, dispersions prepared in this way can only be stored for limited periods because of the gradual reaction of the free NCO-groups with the water. Additional problems are involved in isolating the product from the dispersion by filtration and drying since the product is of a fine-grained nature.
To date, there has been no report of dimerization without solvents on a commercial scale. Although it is relatively easy to dimerize small quantities of aromatic diisocyanates without dilution [see, e.g., Russian Chemical Reviews, 41 (9), 1972, page 776; U.S. Pat. No. 2,671,082 and German Auslegeschrift 1,081,895], dimerization in large quantities has generally failed because of the considerable heat of reaction which has to be dissipated and also because of the liquid/liquid-solid/solid phase conversion by which the reaction is accompanied. Thus, for example, 11 kcal/mol are liberated per mol of uretdione diisocyanate produced from 2,4-diisocyanato tolulene. The intense heating of the reaction mixture to temperatures of 100.degree. C and higher results in the substantially quantitative formation of resin-like trimerization products. The trimerization products can no longer be split back into the dimers or monomers.
At one time, it appeared promising to allow the dimerization reaction to take place in thin layers on a conveyor belt (e.g., a steel belt) with cooling. However, tests have shown that the product obtained does not even begin to approach the quality of a uretdione diisocyanate prepared in dilute solution. The product still contains considerable quantities of unreacted monomeric diisocyanate, so that it smells strongly of monomeric diisocyanate. The thus-prepared uretdione isocyanate is also not uniform in its composition, because when the reaction mixture is applied to the conveyor belt even slight differences in layer thickness result in differential heating of the reaction mixture, so that relatively high quantities of isocyanurate (i.e., trimer) are formed. The reaction product can only be removed from the conveyor belt after a prolonged period, and even then only in fragments. As a result, interruptions in the continuous application of monomer are inevitable.